ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 206

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

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Data Transfer
Atmel ATmega48PA/88PA/168PA [Preliminary]
Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit in
the UCSRnB register is set to one. When the Transmitter is enabled, the normal port operation
of the TxDn pin is overridden and given the function as the Transmitter's serial output.
Enabling the receiver is optional and is done by setting the RXENn bit in the UCSRnB register
to one. When the receiver is enabled, the normal pin operation of the RxDn pin is overridden
and given the function as the Receiver's serial input. The XCKn will in both cases be used as
the transfer clock.
After initialization the USART is ready for doing data transfers. A data transfer is initiated by
writing to the UDRn I/O location. This is the case for both sending and receiving data since the
transmitter controls the transfer clock. The data written to UDRn is moved from the transmit
buffer to the shift register when the shift register is ready to send a new frame.
Note:
The following code examples show a simple USART in MSPIM mode transfer function based
on polling of the Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag.
The USART has to be initialized before the function can be used. For the assembly code, the
data to be sent is assumed to be stored in Register R16 and the data received will be available
in the same register (R16) after the function returns.
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag,
before loading it with new data to be transmitted. The function then waits for data to be present
in the receive buffer by checking the RXCn Flag, before reading the buffer and returning the
value.
To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register
must be read once for each byte transmitted. The input buffer operation is identical to normal
USART mode, i.e. if an overflow occurs the character last received will be lost, not the first data
in the buffer. This means that if four bytes are transferred, byte 1 first, then byte 2, 3, and 4, and
the UDRn is not read before all transfers are completed, then byte 3 to be received will be lost,
and not byte 1.
9223B–AVR–09/11